Templated Assembly of Gold Nanoparticles into Microscale Tubules

Templated Assembly of Gold Nanoparticles into Microscale Tubules and Their Application in Surface-Enhanced Raman Scattering. Tie Wang, Rongbo Zheng, ...
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J. Phys. Chem. B 2006, 110, 14179-14185

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Templated Assembly of Gold Nanoparticles into Microscale Tubules and Their Application in Surface-Enhanced Raman Scattering Tie Wang, Rongbo Zheng, Xiaoge Hu, Lixue Zhang, and Shaojun Dong* State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, People’s Republic of China, and Graduate School of the Chinese Academy of Sciences, Beijing, 100039, People’s Republic of China ReceiVed: March 31, 2006; In Final Form: May 25, 2006

We report a simple procedure to assemble gold nanoparticles into hollow tubular morphology with micrometer scale, wherein the citrate molecule is used not only as a reducing and capping agent, but also as an assembling template. The nanostructure and growth mechanism of microtubes are explored via SEM, TEM, FTIR spectra, and UV-vis spectra studies. The incorporation of larger gold nanoparticles by electroless plating results in an increase in the diameter of microtubes from 900 nm to about 1.2 µm. The application of the microtubes before and after electroless plating in surface-enhanced Raman scattering (SERS) is investigated by using 4-aminothiophenol (4-ATP) as probe molecules. The results indicate that the microtubes both before and after electroless plating can be used as SERS substrates. The microtubes after electroless plating exhibit excellent enhancement ability.

Introduction Since the discovery of carbon nanotubes in 1991,1 tubular nanostructures have attracted increasing attention. Because of their special physical and chemical properties, the potential applications of nanotubes in electric devices, sensors, and others have been explored.2 Besides carbon nanotubes, other types of nanotubes, including metals (e.g., Au, Pt, Ag, Pd),3 inorganics (e.g., Eu2O3, ReS2, VOx, TiO2),4 and polymers (e.g., polyaniline, polythiophene, polystyrene),5 have been successfully fabricated. In addition, the great success of the microelectronic industry has been based on the miniaturization of a few basic device elements, in which different types of junctions are needed.6 For this purpose, Y-junction nanotubes with a complex three-point junction have been considered as prototypes.7 Yet, to date, the research for Y-junction nanotubes mainly focuses on the carbon nanotubes. Gold nanoparticles are particularly attractive building blocks for nanotechnology, with application in different areas from biosensors to electronic nanodevices.8 Current investigation has identified the need for greater control over the assemblies of nanoparticles.9 Because the physical properties of nanoparticles can be strongly influenced by surrounding nanoparticles, some new properties often emerge from the aggregates that are distinctly different from those of the corresponding isolated nanoparticles.10 When clusters of metal nanoparticles are placed in close proximity to one another, such as in surface-enhanced Raman scattering (SERS) experiments, the coupling between particles becomes very important. Neighboring nanoparticles can affect the resonance frequency in a distance-dependent manner, allowing electromagnetic energy to propagate along linear arrays.11 The controlled organization of metallic nanoparticles into one-dimensional, two-dimensional, or threedimensional arrays offers a promising route to tailor the flux of surface plasmons.12 Although a variety of gold nanoparticles * Corresponding author. Phone: +86-431-5262101. Fax: +86-4315689711. E-mail: [email protected].

assemblies are now available,13 the formation of nanoparticlesbased architectures with well-definite geometric morphology is still a challenge in the miniaturization of integrated optical devices.14 Self-assembly provides an effective and versatile approach to construct nanoparticles into spatial structures of larger scale. In this study, we demonstrate a self-assembled procedure to prepare microtubes with gold nanoparticles serving as building blocks at room temperature, wherein citrate is used not only as a reducing and capping agent, but also as an assembling template. In addition, larger gold nanoparticles can be incorporated into microtubes by electroless plating. The as-prepared products preserving hollow tubular morphology exhibit singlecrystalline structure. It is found that the microtubes both before and after electroless plating can be used as good SERS-active substrates with 4-aminothiophenol (4-ATP) as test probes. The Raman enhancement ability can increase with an increase of the diameters of gold nanoparticles. Experimental Section Reagents and Materials. Hydrogen tetrachloroaurate(III) trihydrate (HAuCl4‚3H2O, 99.9%), trisodium citrate (99%), and hydroxylamine (NH2OH) were purchased from Aldrich. 4-ATP was obtained from Acros. All chemicals were used without further purification. Instrumentation. The products were imaged by an XL30 ESEM FEG field emission scanning electron microscope (SEM, FEI Co. with 20 kV operating voltage) equipped with energydispersive X-ray (EDX). Transmission electron microscopy (TEM) was performed with a HITACHI H-8100 EM with accelerating voltage 200 kV. The UV-vis spectra were acquired using a Cary 500 UV-visible NTR spectrometer (Varian). X-ray diffraction (XRD) analysis was carried out on a D/Max 2500 V/PC X-ray diffractometer using Cu (50 kV, 250 mA) radiation. Transmission infrared spectra were collected in the transmission mode on a Nicolet 520 FT-IR spectrometer. The Raman

10.1021/jp0620015 CCC: $33.50 © 2006 American Chemical Society Published on Web 06/30/2006

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instrument includes a FT-Raman spectrometer (Thermo Nicolet 960) equipped with an InGaAs detector and a Nd/VO4 laser (1064 nm) as an excitation source. The laser power used was about 400 mW. All FT-SERS were recorded by averaging 512 scans. Preparation of Microtubes with Gold Nanoparticles Serving as Building Blocks. The experimental method was a modification of the conventional citrate reduction of HAuCl4 in water. In a typical synthesis, 70.00 mL of distilled water, 0.50 mL of an aqueous 24.3 mM HAuCl4‚3H2O solution, and 1.30 mL of an aqueous 34 mM trisodium citrate solution were mixed, and subsequently aged at room temperature (about 20 °C). Results and Discussion Immediately after the addition of citrate solution to HAuCl4 solution, the mixed solution appeared to be colorless. After an aged time of 6 h, the initial solution became darkish, indicating the reduction of HAuCl4. After the aging time was prolonged to 24 h, this solution exhibited the characteristic red color of gold nanoparticles. By further extending the aging time to 20 days, the solutions turned blue accompanied by the occurrence of black suspending floccules that were phase-separated from the aqueous media. Finally, the solution became colorless after 30 days. The color changes of the media and the appearance of the black suspending floccules could be visualized with naked eyes and have been traced by the UV-vis absorption method (Figure 1A and B). The color changes are a direct consequence of appearance and assembly of gold nanoparticles. At the beginning, no absorption from 450 to 700 nm can be observed, showing that the particles are not present at that time. A weak and broad absorption band centered at 530 nm, corresponding to the darkish solution, then appears after aging for 6 h. When the solution turns red after 24 h, a strong and blue-shifted plasmon band appears at about 519 nm. The change in the UVvis spectroscopy indicates the gold nanoparticles formation.15 The diameter of the nanoparticles is about 18 nm (TEM image not shown). When the reaction continued, the peak position redshifts from 519 to 524 nm after aging for 7 days. Also mentioned is that the peak shape becomes broad during the reaction, which can be attributed to ineffectual stabilization of citrate for gold nanoparticles resulting in multiple size domains.16 After 20 days, two plasmon resonances, with the transverse plasmon resonance at 529 nm and the longitudinal plasmon resonance at 590 nm, can be seen. The appearance of the plasmon band at longer wavelength results from the formation of nanoparticle aggregates.17 Eventually, almost all of gold nanoparticles are assembled in the form of black floccules, and the media becomes colorless after 4 weeks. The floccules are purified by four cycles of centrifuging the suspension and discarding the supernatant. An optical microscope image of the floccules dried on a glass slide shows that they exhibit a golden color (Figure 1C). Figure 2 shows typical SEM images of the floccules at different magnifications. Figure 2A clearly represents the large quantity of wirelike products obtained via this approach. The typical lengths of these gold wires are about several hundreds micrometers; some of them even have lengths on the order of millimeters. The presence of several Y-junction gold wires is shown in Figure 2B. The tubular morphology is clearly presented in the inset of Figure 2C. The open ends of these gold wires demonstrate the hollow structure of the resulting product. Thus, these wirelike products are gold microtubes. The microtube walls are composed of a multilayer gold nanoparticles (with diameter of around 18 nm) structure, and their thickness ranges from 70 to 200 nm.

Figure 1. (A, B) Time-dependent UV-vis absorption spectrum of the media during the growth process of gold nanoparticles assemblies. (C) Optical microscope image of gold nanoparticles assemblies.

The chemical composition of these microtubes was determined by elemental analysis. The peaks of Au, C, Si, and O are noticed, as shown in Figure 3. The Si peak originates from silicon substrate. Obviously, these microtubes are predominantly made of gold, carbon, and oxygen as demonstrated by EDX analysis and the corresponding elemental map. On the basis of the elemental composition of microtubes, we can conclude that the microtubes are gold/citrate hybrid. The microtubes are composed of continuous, multilayer nanoparticle arrays and citrate template. Insight into the gold/ citrate hybrid structure can be obtained by TEM. The nanoparticle coverage on the surfaces of citrate template increases with

Assembly of Gold Nanoparticles into Tubules

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Figure 3. EDX analysis of microtubes in a selected domain with silicon slides serving as substrate. Comparison of morphological image and elemental map shows that the microtubes are composed of gold nanoparticles and citrate.

Figure 2. SEM images of microtubes at different magnifications. The inset of image C shows the cross section of microtubes.

increasing self-assembly time. Figure 4A shows a TEM image of microtubes that were aged for 20 days at room temperature. The citrate template with diameter of 500-600 nm is clearly represented. The gold nanoparticles with average diameter of 18 nm are adsorbed on the citrate template, and some fused gold nanoparticles are also seen. In the case of relatively longer aging time, the assembled nanoparticles exhibit a higher packing density, and a multilayer feature is evident. Resulting from the further overlapping of particles, the diameters of microtubes increase from 700 nm (Figure 4B) to 900 nm (Figure 4C). The microtube walls exhibit a polycrystalline nature, as determined by the microscopic electron diffraction (ED) pattern (Figure 4D). The typical wide angle XRD pattern of the microtubes presents five sharp diffraction peaks corresponding to the (111), (200), (220), (311), and (222) facet of face-centered cubic (fcc) gold, indicating the formation of highly crystalline gold (Figure 5A). We then identify the functional groups presented on the gold nanoparticles and microtubes by FTIR spectra (Figure 6A). As for the products after aging 1 day, two major peaks centered at

Figure 4. TEM images of microtubes with gold nanoparticles serving as building blocks: (A) aged time of 20 days, (B) aged time of 25 days, (C) aged time of 30 days, and (D) electron diffraction of image C.

1591 and 1394 cm-1 could be attributed to asymmetric and symmetric stretching vibrations of COO- groups.18 The bands

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Figure 5. (A) XRD pattern and (B) UV-vis absorption spectrum of microtubes.

in the range ν˜ ) 1000-1300 cm-1, which include the C-OH stretching and OH banding vibration, imply the existence of large numbers of residual hydroxy groups.19 In the case of microtubes, the O-H stretching band centered at 3300 cm-1 and stretching vibrations of COO- groups are weakened. Three new bands that appeared at ν˜ ) 1746, 1160, and 1059 cm-1 are attributed to CdO, C(O)-O, and O-C vibrations, respectively.18 It suggests that the final products include citrate ester. From the experiments, the pure citrate template was not observed in the same concentration of citrate solution with the absence of gold nanoparticles, but could be obtained in higher concentration. The FTIR spectra of pure citrate template also indicated the existence of ester groups. Detailed results will be published in a future paper. Because the catalysis of gold nanoparticles for various reactions has been well demonstrated previously,20 we reasonably infer that the esterification of citrate was catalyzed by gold nanoparticles to form a tubular template during the aging process (Figure 6B). Next, the gold nanoparticles are immobilized on the citrate template. Although the fundamental basis of these reactions in this system is needed to further understand, it is important to point out that the citrate template may play a major role in determining the structures of the gold/ citrate hybrid. Considering the absence of any linking molecules, such as bisthiol molecule, to interact the gold colloids, the multilayer nanoparticle aggregation on the citrate template is assumed to involve partial stripping of the citrate stabilizing shell from the

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Figure 6. (A) FTIR spectra of the products after aging 1 day and 30 days, respectively. (B) Schematic model of citrate ester.

Figure 7. TEM images of microtubes after being added in additional citrate solution. The inset is high-magnification TEM images.

surfaces of gold nanoparticles.21 To verify the decisive effect of citrate on the multilayer nanoparticles aggregate, the asprepared microtubes were added in an additional citrate solution. The TEM images are shown in Figure 7. Because citrate molecules readsorb to surfaces of gold nanoparticles, the presence of an electrostatic barrier results in a repulsive interaction.22 The initial multilayer nanoparticles adsorbed on the citrate template are seen to be separated from each other, and simultaneously a large number of nanoparticles escape into solution from citrate template. Because of the coalescence of

Assembly of Gold Nanoparticles into Tubules nanoparticles, the majority of particles are not discrete in the solution (the inset of Figure 7). Based on the above investigations, the formation process is described as follows: First, citrate-stabilized gold nanoparticles are formed by citrate reduction of HAuCl4, in which citrate not only acts as a reducing agent but also as a capping agent. Second, citrate is esterified and transformed into a tubular template with the gold nanoparticles catalysis. The partial stripping of the citrate stabilizing shell from the gold nanoparticle surfaces results in the decreasing surface potential, in which nanoparticles with insufficient capping of citrate stick together, and slightly fuse.23 Because of the interaction of particles, the solution renders a blue color.24 Meanwhile, gold colloids become unstable and are predominantly adsorbed onto the surface of citrate template to form microtubes. Finally, with the elapsed time, more nanoparticles are adsorbed on the citrate template, accompanied by the increasing diameter of microtubes. In addition, a spot of nanoparticle coagulation is also observed in solution. Figure 5B shows characteristic absorption spectra of the microtubes (Figure 4C). The two plasmonic bands are observed at 535 and 685 nm. The two spectral bands were contributed from gold nanoparticles in different domain structures. Actually, for a particle pair, two different aggregate excitation modes can be distinguished, either parallel or perpendicular relative to the pair axis. In the quasistatic approximation, the excitation of the longitudinal in-phase oscillation differs drastically from that of isolated particles, but the transversal one is only slightly redshifted, which is similar to the absorption of isolated particles.25 The longitudinal and transversal absorption bands of a manyparticle aggregate are even more separated than for a particle pair.25 In the case of the close-packed nanoparticle aggregates, the isolated-particle approximation breaks down and electromagnetic interactions between the interparticles become important and affect the optical spectra enormously. Therefore, it is very difficult to observe the characteristic absorption spectra of the isolated nanoparticle.9a The plasmonic frequency in nanoparticle aggregates depends on nanoparticle spacing.24 For gold nanoparticle aggregates with interparticle distances substantially greater than the average particle diameter, the plasmonic bands exhibit absorption of isolated particles, but as the interparticle distances in the aggregates decrease to less than the average particle diameter, a red-shifted band occurs.26 In our experiments, the absorbance band at λ ) 685 nm is a result from the strong electromagnetic coupling between close-spaced nanoparticles on the microtube walls; on the other hand, the appearance of an absorbance band at λ ) 535 nm is similar to the native gold nanoparticles band, which implies the presence of loose-spaced nanoparticles aggregated on the microtube walls. The microtubes are very suitable as SERS substrates because they possess three important characteristics for SERS signal enhancement. First, the nanoparticles attached on the citrate template exhibit a rough surface on the nanometer scale, which is very desirable for enhancement of Raman scattering;27 second, the aggregate of noble metal particles is a prerequisite for strong SERS enhancement;28 and, third, as compared to flat substrates, the presence of loose-packed nanoparticles on the microtube walls increases the accessible surface area for there to be detected molecules.29 In addition, the surface-enhanced effect of the Raman scattering greatly depends on the size of the nanoparticles, because it is directly related to the nanometer scale roughness.30 According to the report of Nie et al., the 60 nm gold particles show efficient SERS intensity enhancement.31 Therefore, incorporating nanoparticles with larger sizes into

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Figure 8. (A) SEM and (B) TEM images of microtubes after electroless plating. The inset of image B is the corresponding electron diffraction.

microtubes is of importance for the application of such materials in SERS. The microtubes (Figure 4C) were immersed in 6 mL of a gold plating solution consisting of aqueous 0.4 mM NH2OH and 0.1% HAuCl4 for 20 min with stirring. The initial gold nanoparticles constructing microtubes serve as catalysts that accelerate reduction of HAuCl4 by NH2OH.32 All particle nucleation only occurs on the surface of existing gold nanoparticles, which ensures the growth of the gold nanoparticles.17 The microtubes after electroless plating were purified by four cycles of centrifuging the suspension and discarding the supernatant. The size of gold nanoparticles constructing microtubes after electroless plating increases from about 18 to around 60 nm, and the tubular morphology is preserved (Figure 8A). The inter-nanoparticles coupling of building blocks of microtubes after electroless plating becomes stronger than that of microtubes before electroless plating. The diameters of the microtubes after electroless plating enlarge to 1.2-1.5 µm (Figure 8B). The ED pattern of the microtubes indicates the single-crystalline structure (the inset of Figure 8B). The SERS intensity of the adsorbates on the surfaces of microtubes before and after electroless plating was investigated. 4-ATP was selected as the probing molecule. The microtubes before and after electroless plating were directly dropped on the ITO substrates, respectively. Subsequently, the ITO glass slides with microtubes before and after electroless plating were immersed in 1.0 × 10-6 M 4-ATP solution for several hours. After being thoroughly rinsed with water and dried by nitrogen, they were subjected to Raman characterization by a FT-Raman spectrometer. For comparison, a gold nanoparticles film was prepared by directly dropping 50 µL of citrate-stabilized 18 nm

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Wang et al. atom. Considering the size of microtubes before and after electroless plating (around 1 µm in diameter) is comparable to that of the laser spot (1 µm in diameter), it is possible that a single microtube before and after electroless plating is just fit to serve as a SERS-active substrate, which may have potential application in future miniaturized nanodevices. Conclusion

Figure 9. SERS spectra of 4-ATP on different substrates: (a) a 18 nm gold nanoparticles film; (b) microtubes before electroless plating; and (c) microtubes after electroless plating.

gold nanoparticles onto planar ITO substrates. A 50 µL drop of 1.0 × 10-6 M 4-ATP solution was coated on the nanoparticles film for drying. Figure 9 displays the FT-SERS spectra of these three samples under 1064 nm excitation. SERS spectra are observed in all cases. The nanoparticles film exhibits only a very weak enhancement. As for microtubes before electroless plating, the SERS signals obtain obvious improvement. The most intense SERS spectrum is shown on the microtubes after electroless plating, accompanied by little background noise, and all of the bands characteristic for 4-ATP that appeared on the rough silver substrate can clearly be observed,33 even though there is a slight frequency shift relative to the literature. Therefore, the microtubes after electroless plating may be an appropriate Raman-active substrate. The three spectra are all dominated with the a1 vibrational modes (in-plane, in-phase modes), such as υ(CC) at 1580 and υ(CS) at 1073 (cm-1).34 The band at 387 cm-1 can be assigned to one of the vibrational modes of the C-S bond, most likely the bending mode of the C-S bond.33 The predominance of a1 modes in the FT-SERS spectrum may imply that the enhancement via an electromagnetic (EM) mechanism is significant. In the case of microtubes before and after electroless plating, the high local EM field originates from the strong inter-nanoparticles coupling among gold nanoparticle aggregate, and therefore the strong SERS signal was observed. After the enlargement of nanoparticles, the decreased interparticle spacing may bring on a further redshift of the plasmonic band,9a,17 with the result that the SERS excitation approaches the plasmonic band of the gold nanoparticle aggregate. The high SERS intensities generally occur when the exciting laser line overlaps the longitudinal surface plasmon resonance,35 a condition under which the EM enhancement mechanism is operative. Therefore, the microtubes after electroless plating show a dramatic enhancement in the SERS activity.29 Furthermore, the SERS enhancement ability is related to the size of the nanoparticles. The nanoparticles films with a larger diameter of nanoparticles enhance the Raman scattering effectively.30 It is noteworthy that the enhancement of b2 modes (in-plane, out-of-phase modes) located at 1428, 1386, and 1135 cm-1 is also apparent. The apparent enhancement of the b2 modes has been ascribed to the charge transfer (CT) of the metal to the adsorbed molecules.36 The enhancement of b2 modes indicates that the ring plane of adsorbed 4-ATP molecules is perpendicular to the gold nanoparticles.33a Therefore, 4-ATP is attached to the gold nanoparticle surfaces just through its sulfur

In summary, a simple and effective procedure for preparation of microtubes with gold nanoparticles serving as building blocks is presented. The process involves the formation of citrate template and spontaneous absorption of nanoparticles, followed by aggregation of nanoparticles to yield microtubes with multilayer gold nanoparticles. The microtube walls are composed of two kinds of multilayer nanoparticle domain structures, that is, close-packed and loose-packed nanoparticle domains. The larger gold nanoparticles of about 60 nm can be incorporated into microtubes by electroless plating. The microtubes after electroless plating exhibit excellent SERS ability. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Nos. 20575064 and 20427003). References and Notes (1) Iijima, S. Nature 1991, 354, 56. (2) Ajayan, P. Chem. ReV. 1999, 99, 1787. (3) (a) Lahav, M.; Sehayek, T.; Vaskevich, A.; Rubinstein, I. Angew. Chem., Int. Ed. 2003, 42, 5576. (b) Hanaoka, T.; Kormann, H.; Kro¨ll, M.; Sawitowski, T.; Schmid, G. Eur. J. Inorg. Chem. 1998, 807. (c) Li, Z.; Chung, S.-W.; Nam, J.-M.; Ginger, D.; Mirkin, C. Angew. Chem., Int. Ed. 2003, 42, 2306. (d) Oshima, Y.; Koizumi, H.; Mouri, K.; Hirayama, H.; Takayanagi, K. Phys. ReV. B 2002, 65, 121401. (e) Qu, L.; Shi, G.; Wu, X.; Fan, B. AdV. Mater. 2004, 16, 1200. (f) Steinhart, M.; Jia, Z.; Schaper, K.; Wehrspohn, R.; Go¨sele, U.; Wendorff, J. AdV. Mater. 2003, 15, 706. (4) (a) Wu, G.; Zhang, L.; Cheng, B.; Xie, T.; Yuan, X. J. Am. Chem. Soc. 2004, 126, 5976. (b) Brorson, M.; Hansen, T.; Jacobsen, C. J. Am. Chem. Soc. 2002, 124, 11582. (c) Krumeich, F.; Muhr, H.; Niederberger, M.; Bieri, F.; Schnyder, B.; Nesper, R. J. Am. Chem. Soc. 1999, 121, 8324. (d) Hoyer, P. Langmuir 1996, 12, 1411. (5) (a) Martin, C. Chem. Mater. 1996, 8, 1739. (b) Feng, L.; Li, S.; Li, H.; Zhai, J.; Song, Y.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2002, 41, 1221. (c) Fu, M.; Zhu, Y.; Tan, R.; Shi, G. AdV. Mater. 2001, 13, 1874. (d) Obare, S.; Jana, N.; Murphy, C. Nano Lett. 2001, 1, 601. (6) Satishkumar, B.; Thomas, P. J.; Govindaraj, A.; Rao, C. Appl. Phys. Lett. 2000, 77, 2530. (7) (a) Gothard, N.; Daraio, C.; Gaillard, J.; Zidan, R.; Jin, S.; Rao, A. Nano Lett. 2004, 4, 213. (b) Wang, X.; Lu, J.; Xie, Y.; Du, G.; Guo, Q.; Zhang, S. J. Phys. Chem. B 2002, 106, 933. (8) Claridge, S.; Goh, S.; Fre´chet, J.; Williams, S.; Micheel, C.; Alivisatos, A. Chem. Mater. 2005, 17, 1628. (9) (a) Wessels, J.; Nothofer, H.; Ford, W.; Wrochem, F.; Scholz, F.; Vossmeyer, T.; Schroedter, A.; Weller, H.; Yasuda, A. J. Am. Chem. Soc. 2004, 126, 3349. (b) Mayer, C.; Neveu, S.; Cabuil, V. AdV. Mater. 2002, 14, 595. (10) Johnson, R.; Lemon, B.; Hupp, J.; Feldheim, D. J. Am. Chem. Soc. 2000, 122, 12029. (11) (a) Storhoff, J. J.; Lazarides, A. A.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L.; Schatz, G. C. J. Am. Chem. Soc. 2000, 122, 4640. (b) Su, K. H.; Wei, Q. H.; Zhang, X.; Mock, J. J.; Smith, D. R.; Schultz, S. Nano Lett. 2003, 3, 1087. (12) Lin, S.; Li, M.; Dujardin, E.; Girard, C.; Mann, S. AdV. Mater. 2005, 17, 2553. (13) Wang, T.; Zhang, D.; Xu, W.; Li, S.; Zhu, D. Langmuir 2002, 18, 8655. (14) Sudeep, P.; Shibu, J. S.; Thomas, K. J. Am. Chem. Soc. 2005, 127, 6516. (15) (a) Frens, G. Nat. Phys. Sci. 1973, 241, 20-22. (b) Chow, M.; Zukoski, C. J. Colloid Interface Sci. 1994, 165, 97. (16) (a) Gao, J.; Bender, C.; Murphy, C. Langmuir 2003, 19, 9065. (b) Slocik, J.; Wright, D. Biomacromolecules 2003, 4, 1135. (17) Brown, K.; Natan, M. Langmuir 1998, 14, 726. (18) Li, R.; Fan, G.; Qu, R. Spectral Analysis of Organic Structures; Tianjing University Press: Tianjing, 2002; Chapter 3. (19) Sun, X.; Li, Y. Angew. Chem., Int. Ed. 2004, 43, 597.

Assembly of Gold Nanoparticles into Tubules (20) Daniel, M.-C.; Astruc, D. Chem. ReV. 2004, 104, 293. (21) Chah, S.; Fendler, J.; Yi, J. J. Colloid Interface Sci. 2002, 250, 142. (22) Biggs, S.; Mulvaney, P.; Zukoski, C.; Grieser, F. J. Am. Chem. Soc. 1994, 116, 9150. (23) Pei, L.; Mori, K.; Adachi, M. Langmuir 2004, 20, 7837. (24) (a) Pal, A.; Ghosh, S.; Esumi, K.; Pal, T. Langmuir 2004, 20, 575. (b) Link, S.; Furube, A.; Mohamed, M.; Asahi, T.; Masuhara, H.; El-Sayed, M. J. Phys. Chem. B 2002, 106, 945. (25) Schmitt, J.; Ma¨chtle, P.; Eck, D.; Mo¨hwald, H.; Helm, C. Langmuir 1999, 15, 3256. (26) (a) Musick, M.; Keating, C.; Lyon, L.; Botsko, S.; Pen˜a, D.; Holliway, W.; McEvoy, T.; Richardson, J.; Natan, M. Chem. Mater. 2002, 12, 2869. (b) Musick, M.; Keating, C.; Keefe, M.; Natan, M. Chem. Mater. 1997, 9, 1499. (27) (a) Li, X.; Xu, W.; Zhang, J.; Jia, H.; Yang, B.; Zhao, B.; Li, B.; Ozaki, Y. Langmuir 2004, 20, 1298. (b) Li, X.; Zhang, J.; Xu, W.; Jia, H.; Wang, X.; Yang, B.; Zhao, B.; Li, B.; Ozaki, Y. Langmuir 2003, 19, 4285. (c) Lu, L.; Zhang, H.; Sun, G.; Xi, S.; Wang, H.; Li, X.; Wang, X.; Zhao, B. Langmuir 2003, 19, 9490. (28) (a) Wei, G.; Wang, L.; Liu, Z.; Song, Y.; Sun, L.; Yang, T.; Li, Z. J. Phys. Chem. B 2005, 109, 23941. (b) Hu, J.; Zhao, B.; Xu, W.; Fan, Y.;

J. Phys. Chem. B, Vol. 110, No. 29, 2006 14185 Li, B.; Ozaki Y. J. Phys. Chem. B 2002, 106, 6500. (c) Schwartzberg, A. M.; Grant, C. D.; Wolcott, A.; Talley, C. E.; Huser, T. R.; Bogomolni, R.; Zhang, J. Z. J. Phys. Chem. B 2004, 108, 19191. (d) Nikoobakht, B.; ElSayed, M. A. J. Phys. Chem. A 2003, 107, 3372. (29) Lu, L.; Randjelovic, I.; Capek, R.; Gaponik, N.; Yang, J.; Zhang, H.; Eychmu¨ller, A. Chem. Mater. 2005, 17, 5731. (30) Suzuki, M.; Niidome, Y.; Kuwahara, Y.; Terasaki, N.; Inoue, K.; Yamada, S. J. Phys. Chem. B 2004, 108, 11660. (31) Krug, J. T., II; Wang, G. D.; Emory, S. R.; Nie, S. J. Am. Chem. Soc. 1999, 121, 9208. (32) Cheng, W.; Dong, S.; Wang, E. Langmuir 2002, 18, 9947. (33) (a) Zheng, J.; Li, X.; Gu, R.; Lu, T. J. Phys. Chem. B 2002, 106, 1019-1023. (b) Zheng, J.; Zhou, Y.; Li, X.; Ji, Y.; Lu, T.; Gu, R. Langmuir 2003, 19, 632. (34) Hu, X.; Cheng, W.; Wang, T.; Wang, Y.; Wang, E.; Dong, S. J. Phys. Chem. B 2005, 109, 19385. (35) (a) Orendorff, C.; Gole, A.; Sau, T.; Murphy, C. Anal. Chem. 2005, 77, 3261. (b) Leopold, N.; Lendl, B. J. Phys. Chem. B 2003, 107, 5723. (36) Osawa, M.; Matsuda, N.; Yoshll, K.; Uchida, I. J. Phys. Chem. 1994, 98, 12702.